In recent years improvements have been made in the free and fixed ammonia still system for treating waste streams from coal conversion facilities to remove ammonia and acid gases from these streams. In the past it was common to pass the vapor stream containing ammonia from the top of the second distillation (fixed still) through the bottom of the first distillation (free still) as a means of conserving energy by using the heat from this vapor stream to heat the liquids in the first distillation. However, it was found that improved performance can be achieved by using an indirect heat exchanger to transfer the heat from the second distillation vapors to the first distillation liquids. This results in a bottom liquid from the first distillation having a lower pH, less than about 8.0, than would result when the heat transfer takes place by direct contact. This lower pH in turn results in improved removal of acid gases from the waste stream being treated in the first distillation. This improved removal of acid gases also results in a purer ammonia vapor stream leaving the top of the second distillation.
One problem that has arisen with this improved ammonia removal system is that the condenser used to concentrate the ammonia in the vapor stream from the first distillation can suffer from serious corrosion problems if relatively high ratios of acid gases to ammonia occur in the feed water. This causes poor performance of the process and increases shutdown time and capitol costs due to more frequent replacement of the condenser as compared to the older system, or the need to employ more expensive materials of construction in the condenser and associated equipment.